1. Field of the Invention
The present invention relates to a projection display apparatus.
2. Description of the Related Art
A conventional projection display apparatus employs a lamp light source such as an extra-high pressure mercury lamp and a metal halide lamp. However, such a lamp light source has a relatively short life and requires frequent replacement. In addition, in a projection display apparatus employing the above lamp light source, an optical system for filtering a red light, a green light, and a blue light from a white light is necessary, which complicates a structure of the apparatus and decreases light use efficiency.
To cope with such inconveniences, a projection display apparatus that employs a laser light source, such as a semiconductor laser, has been developed. Because the life of the laser light source is much longer than that of a lamp light source, maintenance is less required for the laser light source. Furthermore, because a laser light of the laser light source can be directly modulated, the light use efficiency increases and the structure of the apparatus can be simple. In addition, employing the laser light source can lead to a reproduction of colors in a wider.
However, a high coherence of the laser light source causes a speckle, a speckle noise, and a speckle pattern, i.e., a high-contrast spot pattern formed on a screen, which degrades the quality of an image displayed on the screen. For this reason, it is of importance to reduce the speckle and the like in a projection display apparatus employing a laser light source. For example, Japanese Patent Application Laid-open No. H6-208089 discloses a method of rotating a diffusing element, i.e., a diffusing plate, in an optical system to reduce a speckle and the like.
As such a diffusing plate, a frosted glass is widely used. Japanese Patent Application Laid-Open No. H6-208080, however, does not specify where the diffusing plate should be positioned or in which way the diffusing plate should rotate, which does not lead to effective reduction of the speckle. In addition, a mechanism for rotating the diffusing plate is necessary in the above method, which increases the size of the apparatus, the number of components, and the cost, possibly generating a noise.